One way to increase engine efficiency is to reduce engine displacement and boost the engine. However, placing a turbine in an exhaust system can increase engine emissions and reduce engine efficiency during an engine start. In particular, engine emissions can be increased since the turbocharger may sink engine exhaust heat during an engine start rather than passing the exhaust heat to a catalyst to promote oxidation and reduction of exhaust gas constituents. Further, engine efficiency can be reduced when spark is retarded or air mass flow through the engine is increased to warm the mass of the turbocharger. Thus, engine boosting can make it more difficult to meet engine emissions and improve engine efficiency during engine starting.
The inventors herein have recognized the above-mentioned disadvantages and have developed an engine method, comprising: opening a first exhaust valve of a cylinder before a piston of the cylinder reaches BDC compression stroke of the cylinder; directing exhaust gases across the first exhaust valve into a first conduit; recovering heat from the exhaust gases in the first conduit to a liquid; and returning the exhaust gases to a second conduit that is in communication with a second exhaust valve of the cylinder.
Engine efficiency can be increased while engine emissions are reduced by separating the exhaust ports of a cylinder and separately processing blow down (e.g., expanding exhaust gases in a cylinder before time when a piston of the cylinder reaches bottom dead center expansion stroke) and residual exhaust gases (e.g., gases that remain in the cylinder after blow-down). In particular, exhaust energy can be transferred from the blow-down gases to operate a turbocharger or to reduce engine friction by quickly warming the engine via an exhaust heat recovery device such as a gas-to-liquid heat exchanger. At the substantially same time, residual gases are directed from a second exhaust port of the cylinder to heat a catalyst, thereby reducing engine emissions. In this way, exhaust gases can be used more efficiently than simply directing all the exhaust gas of a cylinder to a turbocharger.
The present description may provide several advantages. For example, the approach may improve fuel economy and reduce particulate emissions by decreasing engine warm-up time. Further, the method can reduce engine emissions since at least a portion of cylinder exhaust gases are directly routed from the cylinder to the catalyst. Further still, the average exhaust gas pressure supplied to the turbocharger can be increased to improve turbocharger output.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.